Earlier today I was working on a project that I very much want to finish ASAP but it is one of those things where you have to get a frame render preview(s) before you decide to do a final Cinema 4D render.

So today I've finally reached that point and I'm starting to render my preview frames and going through the settings to optimize between the factors of time and quality.

haven't seen anything like this in 15 unless settings in GI are changed or AO is added etc. but with your situation where nothing has changed.... it might help to see the scene (if you are allowed post it).